Behaviors Of Mold-Filling And Solidification For Vertical Centrifugally Cast TC4 Alloys

Microstructures and mechanical properties of vertical centrifugally cast TC4 alloys were experimentally investigated with the computational simulation as an assistant analysis method. The mold-filling and solidification behaviors of vertical centrifugally cast titanium alloy were researched with simulation method. The results were obtained:Casting simulation software of LXCASM-3D developed by our research group was improved in this study. The mold-filling and solidification behaviors of vertical centrifugally cast titanium alloys with simple plate configuration and in multi-groups of thin-walled castings were investigate by the software, and the proper casting techniques were proposed in the present work. The simulation results show that, the mold-filling and solidification behaviors of the castings, directly connected to the vertical sprue and of a large horizontal section area in a vertical centrifugal-casting system, may be highly sensitive to Coriolis forces. To avoid the possible related casting-defects forming, a more complex gating system that can avoid the direct casting-cavity-filling occurring should be designed. A"turn-back"mold-filling technique, which takes advantage of the centrifugal force in a transient rotating melt system, has been shown to be a rational vertical centrifugal-casting process, in order to achieve a smooth and layer-by-layer mold-filling control.Effects of mold rotating rate and centrifugal force on inner defects and stepped casting dimension shrinkage in length dirction were investigated for the vertical centrifugally cast TC4 alloy in graphite mold with wedge and stepped castings. The results show that, the stepped casting shrinkage-cavity amount decreases in a manner of exponential decay with the increasing of centrifugal force, and no obvious macropores were found when the gravity coefficient is largr than 15. The microdefects, e.g. the relative amount of micropores or microcracks appear decreasing trend, however, the inclusions amount increases significantly with the mold-rotating rate increasing. Mold-filling process is another important factor that influences the casting qualities. The casting parts with an early and longer mold-filling path will have more defects because more inclusions and gas bubbles may be easily entrapped into the alloy melt. The stepped casting dimension shrinkage decreases obviously with the rotating rate increasing. At a given mold rotating rate, the mold cooling capbility shows more significant effect on dimension shrinkage, compared with centrifugal forces. Effect of casting modulus/cooling rate and centrifugal force on the structures and mechanical properties were experimentally investigated for the vertically centrifugal casting of TC4 alloy in graphite mold with wedge and stepped casting cavities. The results show that, the microstructures of TC4 castings formed under centrifugal forces still exhibit the lamellar structures. The grain and lamellar thickness can be refined with increasing of centrifugal force or decreasing of casting modulus/increasing of cooling rate and the tensile and yield strengths of TC4 castings can be enhanced. The empirical relationships among the casting process parameters, structural sizes and mechanical properties were achieved.To be the comparison of titanium alloy cast in graphite mold at different mold rotating rates, the structures and mechanical properties of TC4 alloy with steeped casting in steel mold under gravity forces and with multi-group thin walled castings in cramic shell mold under centrifugal forces were investigated. Changing trend of structures with the cooling rates for the alloy cast in gravity with steel mold is similar to that in graphite mold. Therefore, it is indicated that, cooling rate is the main influential factor that affects the structures while the influence of composition is limited. Thus, these experimental data from the two different-mold materials may be reasonably put together to characterize the correlations between the structural dimensions and cooling rates. For the alloy cast in the ceramic-shell mold, when the cast modulus M is less than 7.00mm, the modulus has a significant influence on the cooling rate. However, when M is larger than 7.00mm, the effect of modulus on the cooling rate is weakened. Microstructures of centrifugal TC4 castings both in ceramic and graphite molds exhibit similar lamellar structures. The grain size,α/βlamellar thickness of castings increase with the increasing of casting module, while the tensile strength varies in an opposite changing manner in the two molds when the castings formed under the same centrifugal forece. The effects of casting modulus on the structures and mechanical properties in ceramic mold are more remarkable.